A deadly game of marine chess began in the Southern Ocean off Antarctica yesterday after environmentalists closed in on a Japanese fleet that has been sent to kill 1,000 whales.

After a 10-day search, contact with the Japanese fleet was made at 2.30am yesterday when the Greenpeace ship, the Esperanza, picked up the Japanese ships' distinctive echoes on its radar.

'We are now chasing the fleet's mother ship, the Nisshin Maru, at about 15 knots,' Greenpeace's Sara Holden told The Observer from the Esperanza. 'In turn, we are being followed by one of the fleet's catcher vessels. The weather is clear at present, but we have run into banks of fog, and of course there are icebergs in these waters.'

The Esperanza's crew has pledged to harry the Japanese fleet and block any attempts to harpoon the minke and fin whales it has been sent to catch for what they claim is 'scientific research'.

The cat-and-mouse chase in the Southern Ocean has been further confused because a separate group of activists, who have promised to ram ships trying to catch whales, is also closing in on the whalers, raising fears that the battle between the Japanese fleet and green groups - now in its third year - could lead to loss of life this time.

'We are not down here to protest,' said Paul Watson, of the militant group Sea Shepherd International. 'We are here to stop them.' Last year Watson's ship, Steve Irwin, named after the Australian conservationist, collided with a Japanese whaling vessel, although no serious damage was done to either craft. The Japanese have branded Watson an 'eco-terrorist'.

However, Greenpeace has refused to co-operate with Sea Shepherd or give away the co-ordinates of the Japanese fleet, despite pleas by Watson that the two groups should join together to fight the whalers. 'We have always pledged to take a non-violent approach to saving the whale,' said Holden. 'We are not going to compromise on those ideals and we are not going to help people who have said they will use violence. We are here to save the whale, not put the lives of whalers at risk.'

Watson yesterday promised that his spies within Greenpeace would soon provide him with the location of the Japanese fleet. 'We have our sources in Greenpeace. There are quite a few disgruntled Greenpeacers who are opposed to its policy of non-cooperation. They are being very helpful.'

Last night there were still no signs that Sea Shepherd was close to the Japanese. Esperanza is carrying 35 Greenpeace activists, four inflatable boats and a large amount of fuel, enough to maintain its pursuit for several weeks, it is claimed. 'The Japanese cannot hunt whales while they are running from us, so if we keep up our pursuit we can stop them killing whales for as long as our fuel lasts,' added Holden. 'Morale is very good on board. Things are very upbeat now that we are after the Japanese fleet.'

Japan dispatched its whaling fleet in November with orders to kill about 1,000 whales - 935 minke and 50 fin -under a programme that Tokyo says is for scientific purposes, but which anti-whaling nations and activists say is a front for commercial whaling.

However, under worldwide pressure, Japan last month abandoned its plan to include 50 humpback whales in this season's hunt. That would have been the first major hunt of humpback whales since the 1960s.

Japan's whaling fleet is run by a government-backed research institute and operates under an International Whaling Commission clause that allows the killing of whales for scientific purposes. Critics say the programme is a shield for Japan to keep its whaling industry alive until it can overturn the 1986 ban.

The current Japanese hunt is due to last until April. Last year's was cut short after a fire damaged the Nisshin Maru. In previous years the whalers have been able to avoid environmentalists for weeks, evading them in their faster ships. In 2006 the Greenpeace ship Arctic Sunrise collided with a Japanese whaler during a tense standoff over whales, causing minor damage but no injuries.

Yesterday the Australian government urged both sides to take care. 'The people actually at the site, on the high seas, need to be very careful,' Assistant Treasurer Chris Bowen told reporters in Sydney. 'They need to exercise restraint because their own personal safety is at risk and the personal safety of others is at risk.'